1. Field of the Invention
The present invention relates to liquid crystal display (LCD) devices, and in particular to substrate structures for LCD devices and fabrication methods thereof.
2. Description of the Related Art
Liquid crystal display (LCD) devices have many advantages such as small volume, light weight and low power consumption, and are applicable in a variety of electronic and communication devices including notebook computers, personal digital assistants (PDA), mobile phones and the like, i.e., liquid crystal displays feature lighter weight, thinner profile, and increased portability.
A conventional LCD device includes a pair of substrates with opposing electrodes thereon. A liquid crystal layer is interposed between the pair of substrates. An electric field is applied on the opposing electrodes to control liquid crystal molecular orientations in the liquid crystal layer, thereby displaying desirable images. Two alignment layers are separately interposed between the interfaces between the liquid crystal layer and each substrate, providing initial orientations and pre-tilting status for the molecules in the liquid crystal layer.
From a fabrication point of view, two alignment layers are separately formed on each substrate. Conventional methods provide relief printing or anastatic printing methods to apply polyimide material covering indium tin oxide (ITO) electrodes. Since photo-spacers are formed on a color filter substrate or on an active matrix substrate, the polyimide material also conformably covers the photo-spacers. However, if the alignment layer at the pixel region provides vertical orientation of LC molecules on the substrate, the alignment layer on the side wall of the photo-spacers provides horizontal orientation of LC molecules to the substrate, as shown in FIG. 1.
In FIG. 1, a transparent electrode layer 11 such as indium tin oxide is disposed on a transparent substrate 10. A plurality of photo-spacers 12 are formed on the transparent substrate 10 dividing a plurality of pixel regions. An alignment layer 13 such as polyimide (PI) is disposed on the transparent substrate 10 conformably covering the photo-spacers 12. Since the alignment layer 13 provides liquid crystal molecules having the same orientation and pre-tilting status, the liquid crystal molecules 14a neighboring the photo-spacers 12, i.e., within region b-b, are aligned with the alignment layer 13a on sidewalls of the photo-spacers 12, thereby creating horizontal orientation. The liquid crystal molecules 14 in the pixel region have different alignment orientations, i.e., vertical orientation from the liquid crystal molecules 14a leading to light leakage near the photo-spacers 12.
The above mentioned light leakage due to different alignment orientations adjacent to the photo-spacers may cause low contrast ratio and deteriorate display quality. Accordingly, in order to reduce light leakage, an additional black matrix (BM) area corresponding to the photo-spacers is provided to shield light leakage near the photo-spacers. Although the additional black matrix area can effectively solve the light leakage problem, the additional black matrix area, however, also reduces aperture of the display panel as well as display luminance.
FIGS. 2A-2D are cross sections showing conventional fabrication steps of an alignment layer with different orientation regions on a display substrate. The alignment layer on the sidewalls of the photo-spacers 12 is modified by photochemical reaction, thereby ameliorating the light leakage problem. Japan Patent No. 2004325527, the entirety of which is hereby incorporated by reference, discloses an alignment layer such as fluoride containing silane polymer or polyimide (PI) is applied on a transparent substrate. The alignment layer is pyrophobic (hydrophorbic) with vertically orientated LC molecules. A photo-catalyst mask is used to induce the alignment layer near the photo-spacers creating hydrophilic radicals, thereby creating a hydrophilic alignment region with horizontal alignment orientations neighboring the photo-spacers.
Referring to FIG. 2A, a transparent substrate 2 with a plurality of protrusion structures 12 thereon is provided. For example, a plurality of photo-spacers are formed on a transparent glass to divide a plurality of pixel regions. Next, an alignment layer 20a such as a fluoride containing silane polymer layer is conformably formed on the transparent substrate 2 covering the protrusion structures 12. The alignment layer 20a is pyrophobic (hydrophorbic) with vertical alignment orientations.
Referring to FIG. 2B, a photo-catalyst mask 22 with a transparent region corresponding to the protrusion structures 12 is provided. The transparent substrate 2 is illuminated by UV radiation 26 using the photo-catalyst mask 22 as a shield. The alignment layer 23 near the photo-spacers is photo-chemically reacted generating a hydrophilic radical, thereby creating a hydrophilic alignment region 23a with horizontal alignment orientations neighboring the photo-spacers, as shown in FIG. 2C. The photo-chemically reacted alignment region 23a provides liquid crystal molecules horizontal alignment orientations.
Alternatively, a photo-catalyst can be added to the alignment layer and directly irradiate UV light using a photo-mask as a shield. The alignment layer near the photo-spacers is photo-chemically reacted creating a hydrophilic radical, thereby creating a hydrophilic alignment region 23b with horizontal alignment orientations neighboring the photo-spacers, as shown in FIG. 3. The conventional method, however, requires applying the alignment layer to the entire substrate and the tedious addition of photo-catalyst and photo mask procedures, causing high production cost and low yield.